Gravity dominated two-phase flow in vertical rod bundles

The fluid flow configuration through the core during reflood in a Loss-of-Coolant-Accident in a reactor is investigated. Both, cold leg injection and the cold leg + upper plenum injection are studied. The core behavior is dependent primarily on whether the core in un-flooded, partially-flooded or flooded. This work deals mainly with the un-flooded core. For an un-flooded core, the flow transition from counter-current slug flow to counter-current annular flow is inlet geometry dependent and occurs due to vapor bubble trapping. A method is developed to predict the range of down-flow liquid velocities for which the bubble is trapped. Flow reversal (or dump) through the core occurs if pressure in the upper plenum becomes more than the pressure in the lower plenum. Due to high down-flow liquid velocities in flow reversal, the upper plenum is drained of liquid very quickly and an oscillation down-flow develops. In most of the cases, a one-dimensional analysis, using an average heat flux, provides reasonably accurate predictions